You should see a doctor.

I tend to get it in the last few miles of Olys. Seems sprints are too short, and HIMs not at high enough intensity.

Never had any lingering effects and it stops when I stop.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

You may have exercise induced esotropia, an uncommon condition. Next time you get double vision take a good selfie of your eyes. I would take a video of your eyes rather than just a still jpeg. One or both eyes may be deviated inward resulting in double vision. You need to see an ophthalmologist, preferably one in a major medical center, and should ask to have an MR of your brain. You will also need to have some lab work to check for things like hyperthyroidism and diabetes. What part of the world do you live in? You don't describe what form of exercise you are doing. Swimming with overly tight goggles can result in double vision but given your user ID I would guess this is not swim related.